This invention relates to devices for the treatment of heart disease and particularly to endoarterial prostheses, which are commonly called stents. More particularly, the invention relates to improved metal stents that are coated with expanded polytetrafluoroethylene (ePTFE) in an expandable form.
A focus of recent development work in the treatment of heart disease has been directed to various forms of expandable stents. Stents are generally tube shaped intravascular devices which are placed within a blood vessel to structurally hold open the vessel. The device can be used to maintain the patency of a blood vessel immediately after intravascular treatments and can be used to reduce the likelihood of development of restenosis.
Catheter systems are frequently used to deliver stents to the desired stenotic location. Stents delivered via catheter systems therefore often require extreme flexibility so as to be capable of being transported through varying and tortuous turns and diameters of the vessel pathway prior to arriving at the desired site. Expandable stents are so designed.
Expandable stents are delivered in a collapsed form to the stenotic region and are expanded into the vessel wall thereafter, typically by self expansion properties or by force from an underlying inflated balloon. Most balloon expandable stents can be divided into coil or tubular designs. The tubular stents are usually constructed from a metal tube cut into special pattern, which expands by the force of underlying balloon. The stent typically is crimped on an expandable balloon at the distal end of a catheter assembly by the manufacture or by the operator. After travel in reduced form to the stenotic site, the stent is expanded into the vessel wall by inflating the balloon.
Often referred to as memory stents, self expanding stents may be composed of metals, like nitinol, that are in the elastic or pesudo-elastic range of deformation. Such stents are restrained, typically by a sheath, during travel to the lesion and are sprung open against the vessel wall after the restraint is removed. Since the metal is in its xe2x80x9cspringxe2x80x9d or xe2x80x9cpesudo-elasticxe2x80x9d state, it will continue to apply outwardly supportive force to the vessel wall after the stent has been deployed. These stents are available, for example, in mesh designs or, for example, as a chain of corrugated rings.
Stents can also be configured in lesion specific designs, and include a radio-opague marker or coating, and may be in the form of a stent-graft. Stent-grafts are constructed with a prosthetic vascular graft material (eg, e-PTFE or Dacron). The graft material separates the blood flow from the native luminal surface, which may be atherosclerotic, aneurysmal, and/or injured from angioplasty. Grafts have been employed to cover aneurysms, perforations, and degenerated vein graft lesions.
Stents formed from metallic materials are used for strength and rigidity to aid in holding open the targeted vessel wall. Various metals such as stainless steel 316L, tantalum, platinum, nitinol in its martensite form, and alloys formed with cobalt and chromium have often been used to construct such stents whether in self or balloon-expandable form. Metal stents can be formed in a variety of configurations such as helically wound wire stents, wire mesh stents, weaved wire stents, metallic serpentine stents, or in a chain of corrugated rings.
Metallic serpentine stents, for example, not only provide strength and rigidity once implanted they also are designed sufficiently flexible for traveling through the tortuous pathways of the vessel route prior to arrival at the stenotic site. Additionally, such stents can be made expandable by way of expandable balloon, memory after compression, or otherwise.
Stents that possess a metal surface, however, suffer from a number disadvantages. They may possess burrs, nicks, or sharp ends resulting in insertion and travel resistance.
Also, metallic expandable stents, such as wire mesh and serpentine designs, for example, do not possess uniformly solid tubular walls. Although generally cylindrical in overall shape, the walls of such stents are perforated often in more of a framework design of wire-like elements connected together or in a weave design of cross threaded wire. In either case, the perforated design not only provides expandability, it also provides flexibility for traveling to the stenotic site.
Radial expansion of metal stents usually results in expansion of the spaces between the wires or the struts comprising the perforated stent wall. Such enlarged spaces at the stenotic site provide greater openings for ingrowth of thrombotic material that if great enough may lead to invasion of the flow path and result in restriction of fluid flow or even complete blockage. In addition, a stent with such anchoring ingrowth may prove difficult to remove.
Advantages of employing polytetrafluoroethylene (PTFE) as a stent cover material are well known to those skilled in the art. PTFE is a thermoplastic polymer that is chemically inert, is biocompatable, and has a smooth, flexible, and low-resistance surface to aid the stent insertion procedure. Expanded polytetrafluoroethylene (ePTFE) possesses micropores and may be available in an expandable form. The micropores provide mechanical bonding locations for underlying melt thermoplastics and provide openings on the surface of the cover for limited tissue ingrowth and helpful endoluminal anchoring. The expandable form of ePTFE is expandable facilitating expansion of an underlying metal stent.
ePTFE can be made in a variety of thicknesses, but alone can be insufficiently rigid to hold open a vessel. Stents made of only ePTFE would require relatively thick walls and thus relatively small openings for fluid flow. Moreover, ePTFE possesses elastic properties so that expansion of a stent made from only ePTFE may not remain in an expanded condition after an expandable balloon, for example, is deflated and withdrawn. While offering a smooth low friction surface, other stent materials are often preferred for superior structural performance.
The benefits of partial covering and total encapsulation of a stent body with ePTFE material has been recognized. Covered with expandable ePTFE film, a metal stent can be expandable, biocompatible, and can be sufficiently flexible and present a smooth low-friction surface for endoluminal travel. Such a stent may also be sufficiently structurally rigid to support a vessel wall and encourage beneficial microendotheial growth. Additionally, expandable ePTFE as a cover material can provide unbroken cover before, during, and after expansion of the underlying metallic stent body. The integrity of the ePTFE cover can therefore be maintained during stent expansion to continue shielding and protecting the vessel wall from the underlying metal. Additionally, the stent covering of expandable ePTFE serves to protect the patency of the vessel itself by inhibiting thrombotic growth through the large perforations of the expanded metallic stent body.
It is well known, however, that PTFE, ePTFE, and expandable ePTFE, by virtue of their non-stick properties, can not be easily adhered, whether by mechanical bonding, chemical bonding, or otherwise, directly to a surface. Techniques of sodium etching, mechanical roughening, and plasma treating have been proposed in efforts to improve bonding strength between metal and PTFE material. These processes, however, are not practical to cover stents because adhesion of any of the aforementioned forms of PTFE to the metal surface is relatively weak and because such treatment increases the roughness or polarity of the surface which may then cause undesired trauma or cellular response.
For example, twisting, bending, and expansion of the stent body may cause such bonds to break thus loosening or even dislodging the covering from the stent body. Dislodgment of the stent cover while operational could be troublesome.
Rapid growth in the use of endovascular stents suggests improved clinical outcomes with their use. Coronary stents have significantly improved the outcome of percutaneous treatment of coronary stenoses by reducing immediate complications and long-term restenosis. The stent appears to be an advantageous percutaneous device that effectively treats abrupt vessel occlusion by virtue of its ability to support the vessel wall, prevent intimal flap prolapse into the lumen, and prevent elastic recoil of the arterial wall, thus providing an open vessel where laminar blood flow is maintained and the patency of the arterial lumen is preserved.
There has long existed a need for a metallic stent covered with a secure ePTFE cover. There has also long existed a need for a method to bond thermoplastic polymers to a stent with a metal surface. The present invention satisfies this need.
The invention is directed to an improved stent that has an exterior metal surface which is covered with a primer containing both an active agent for chemical bonding to the metal and a thermoplastic polymer for melting into, and forming a mechanical bond with, a top coating of expandable ePTFE. This invention is also directed to a method for adhering expandable ePTFE to a metal prosthesis.
The improved stent is formed from one or more of a variety of metals including stainless steel, titanium, silver, gold, tantalum, nickel-titanium, manganese, cobalt-chromium, and nickel. Alternatively, the improved stent may be formed with an exterior metal surface from a variety of other materials capable of maintaining its material and mechanical properties after exposure to the temperatures and pressures disclosed herein. Any stent pattern is useable with the present invention coatings.
Applied to the metal surface is a primer containing both a chemically reactive agent for chemical bonding to the metal surface and a thermoplastic polymer for melting into a top coating of thermoplastic polymer. Once applied, the chemically reactive agent of the primer reacts and bonds to the metal surface. The top coating is applied over the primer and the portion of the primer containing the thermoplastic polymer is heated sufficiently to melt forming a mechanical bond with the top coating of expandable ePTFE.
In the event superior adhesion of expandable ePTFE to the stent is desired, the thermoplastic polymer contained in the primer includes fluorinated ethylene propylene (FEP), a copolymer, over which is applied a layer of FEP. Use of primer is necessary as neither FEP nor ePTFE alone adhere adequately to a variety of metals including stainless steel. The over layer of FEP and the primer are then heated to combine with the thermoplastic polymer contained in the primer. The FEP and primer layers are then cooled prior to application of the the ePTFE top coating.
After the expandable ePTFE top coating is applied, the FEP is then heated to above its melting point, but below the melting point of the ePTFE. Not exceeding the melt temperature of the ePTFE preserves the microporous structure and elasticity of the expandable ePTFE coating. With the elevated temperature, pressure is simultaneously applied to the expandable ePTFE to force the melted FEP into the micropores of the expandable ePTFE.
Each of the coatings herein described can be applied by spray, dip, brush-on, plasma deposition, or application of preformed film or any technique known to those skilled in the art. Furthermore, selected portions of the stent may be utilized, for example, for application of one or more of the coatings. In one embodiment, for example, only the annular ring ends of the stent are coated with primer and FEP, and a preformed sleeve of expandable ePTFE is pulled or rolled over the stent body and adhered thereto by application of heat and pressure. Adherence to only the ends of the stent body can provide an outer covering of expandable ePTFE without adherence of the ePTFE to the entire outer surface of the stent body. The unadhered expandable ePTFE film, while not bonded to the stent body, is free to be physically supported by, and expanded in conformance with, the underlying metal stent body.
In another embodiment, expandable ePTFE films encapsulate the entire stent body by bonding inner and outer sleeves of expandable ePTFE to the stent body to wholly cover the stent sealing the metal from exposure to any body fluid or tissue. In yet another embodiment, all surfaces of the stent body, including the surfaces between interstitial gaps in the stent wall, are coated by a covering of primer, FEP, and expandable ePTFE.
In addition, multiple layers of primer, FEP and expandable ePTFE may be applied as desired to achieve desired coating thickness. Expandable ePTFE is preferred for its microporus and expandable properties. Where expansion is not required, ePTFE may be substituted. Where neither expansion nor a superior bond is required PTFE may be substituted. In addition, other melt thermoplastic polymers may be used in lieu of FEP and expandable ePTFE depending upon the performance desired. Furthermore, an ePTFE cover may be adhered to a stent or, similarly, to a medical prosthesis with a metallic surface. Attaching a film of ePTFE in the form of a graft to a metal stent by adhesion as disclosed is also within the scope of the invention.